The present invention relates to a flash control method and apparatus for a variable focal length camera, and more particularly relates to a flash control apparatus for driving two flash units according to the focal length of the camera.
Japanese Unexam. Patent Publ. No. 02-50142 discloses a flash control method for a camera, wherein either a built-in flash or an attached flash is driven to flash prior to an exposure, and the other is driven to flash during the exposure. In this way, the so-called "red-eye" phenomenon is prevented, because the pupils of the subject's eyes contract in response to the first emission preceding the actual exposure. (Hereinafter, such an emission preceding an actual exposure is referred to as a pre-emission, and the next emission during the exposure is referred to as a main-emission.)
A built-in flash unit of a camera ordinarily has a light distribution curve such that the illuminating angle of light therefrom is wide enough to cover the predetermined angle of view of the taking lens of the camera, whereas an external flash unit generally has a light distribution curve such that the illuminating angle of light therefrom is narrower than the light from the built-in flash unit in order to project light toward a more distant range than the built-in flash unit, even when the guide number of the external flash unit is equal to that of the built-in flash unit.
If the above-described known flashing method were applied to a variable focal length camera such as a zoom camera, having an ordinary built-in flash unit and an ordinary external flash unit, and the external flash unit were used for the pre-emission and thereafter the built-in flash unit were used for the main-emission, then the resulting photograph could be under-exposed when the taking lens is set in a long focal length region and the subject distance is relatively large, as is often the case in telephotography. This is because the angle of view of the taking lens set in the long focal length region is so narrow that the light from the built-in flash unit will spread beyond the angle of view and thus the density of light within the photographic field will be reduced.
On the other hand, if the built-in flash unit were used for the pre-emission and thereafter the external flash unit were used for the main-emission, then the peripheral area of the resulting photograph could be under-exposed when the taking lens is set in a short focal length region, because the angle of view in the short focal length region is wider than the illuminating angle of light from the external flash unit, and thus only the central area of the photographic field would be effectively illuminated by the external flash unit.
In order to effectively and uniformly illuminate the whole photographic field in whichever focal length region the taking lens is disposed, it would perhaps be possible to make the guide number of one of the flash units that is used for the main-emission large enough to illuminate a distant subject sufficiently, at the same time as making the illuminating angle of that flash unit wide enough to cover the wider angle of view in the short focal length region. But this approach would require increased capacity of the flash unit for main-emission, such that the volume of either the external flash unit or the built-in flash unit and therefore that of the camera body would be increased.